Proper backfill for a trench dug below foundation level (within the 45 degree cone of support)

Question

Given a trench dug below foundation level, what's the proper way to backfill? Here I'm talking about a trench below the bottom edge of an unreinforced concrete foundation built in 1950, supporting a 2 story stucco house. The trench is somewhat within the 45 degree soil cone supporting the weight of the house:

The foundation itself is about 13 inches high, the trench is 14 inches below that point, set off about 9 inches. Three pipes are in the trench: solid 2", 2" and 1.5" cellular core ABS. The soil is a heavy clay. There is some surface water flow over this area. The home is in an earthquake hazard zone. The original soil grade was much higher: reaching the stucco as shown in the diagram (perhaps 25 inches above the current bottom of trench).

What's the proper way to prevent long term settling (or soil loss) below the foundation in this case? Do you have references? Thanks.

Answer 1

No handy references, (I was taught, as grunt labor, by a licensed civil engineer and former SeaBee, but I don't know which books it might have come from to him, or if any of them are on the web) but backfilling with something like road base (crushed rock including fines so it will pack well) and tamping it very well in thin layers (if you tamp 6" of fill, you only properly compact about 2" of it, so tamping 2" layers 3 times gets a better result.) Tamp with something fairly small - a large plate won't really compact as well (unless you rent a vibratory plate compactor.) If you are tamping by hand and it feels frustratingly slow, you are probably doing it right ;-)

Clay is virtually impossible to get properly compacted back into a trench - in non-bearing applications you generally backfill it and mound it a bit, then come back in year to fix the settling. Obviously not such a good idea in a bearing application, thus the use of a readily compactable material like road base.